c-lightning is a standard compliant implementation of the Lightning Network protocol.
Read all the available documentation at https://lightning.readthedocs.io/.
Esplora is a block explorer web interface based on the esplora-electrs HTTP API. It's written as a single-page app in a reactive and functional style using rxjs and cycle.js.
See live at blockstream.info.
The API documentation is available here.
Wally is a cross-platform, cross-language collection of useful primitives for cryptocurrency wallets.
Read the API documentation at https://wally.readthedocs.io.
GDK is a cross-platform, cross-language library for Blockstream Green wallets.
You can read the API documentation at https://gdk.readthedocs.io/en/latest/.
Elements is an open source, sidechain-capable blockchain platform.
Built upon and extending Bitcoin's codebase, it lets developers quickly and cost-effectively create working blockchains and test proof-of-concept projects. It also enables Elements to function as a testbed for changes to the Bitcoin protocol itself. Blockstream's own production sidechain, Liquid, is built using Elements.
The Elements community website contains:
A walk-through tutorial.
Basic and advanced code samples.
Application development guides.